Process variable transmitters are widely used for sensing or measuring a process variable, and transmitting data so generated to a remote location. A common means for transmitting such data is by means of a current loop, the value of the process variable being represented by the magnitude of a current passing through the loop, the magnitude lying between predetermined minimum and maximum values, typically 4 ma and 20 ma. Such a current loop has a high degree of noise immunity and widespread industrial acceptance.
The transmitter will usually have electrical power requirements of its own, and it is often convenient to meet these power requirements from the current passing in the loop. A limitation of such loop-powered transmitters has been that they must be able to operate at the minimum level of loop current, typically 4 ma.
A characteristic of prior art loop powered devices is that each device must have a separate loop powered circuit. First, as there is no effective method to separate the output from multiple powered devices on a single loop powered circuit. Second, the loop current has a maximum current of 20 ma, and placing multiple devices on the loop would quickly exceed the current limitation, leaving no room to communicate signals via current modulation.
One method to expand use of the legacy 2-40 mA systems is to implement a digital signal on top of the analog loop current. For instance, HART protocol is such an implementation, The HART protocol uses 1200 baud Frequency Shift Keying (FSK) based on the Bell 202 standard to superimpose digital information on the conventional four to 20 mA analogue signal. Both the 4-20 mA current and the digital signal are valid output values from the instrument. The polling address of the instrument is set to “0”. Only one instrument can be put on each instrument cable signal pair, if the instrument is powered from the loop.
In a second HART version, multiple devices can be positioned on a single loop wire pair and the devices are powered through the loop current. In this embodiment, a multidrop environment, the loop current is used to power the devices and to support digital signal communications. FSK is used to address and poll each device, and FSK is used by the device to respond to a request by the controller. In multidrop mode it is possible to have up to 15 instruments on one signal cable. The polling addresses of the instruments will be in the range 1-15, and each meter sensor needs to have a unique address. Each device is polled through its address by a controller to establish communication with the addressed device. Once in communication with a controller, measurement and control of devices within the process is effected, with the device responding via FSK to the controller.
There is a need to be able to use a legacy loop powered circuit to communicate with at least two devices, or to communicate with a single device that can provide multiple readings without using separate power or HART protocol or other digital protocol communications.